The basic concept is related to a tunable IC filter which is composed of a Gilbert cell (an analog multiplier) with a capacitor connected across its current output terminals. This creates an R-C filter which can be tuned by means of the Gilbert cell transconductance. This basic concept was published in the February 1981, issue of ELECTRONIC DESIGN on page 44. The teaching in this publication is incorporated herein by reference.
This basic concept has been expanded to create an analog computer filter that is composed of a large number of cascaded tunable filter elements whose tuning are ganged together and operated from a single control. This filter has evolved into a configuration wherein a plurality of cascaded tunable stages incorporate stabilizing amplifier feedback elements. Since the feedforward and feedback amplifier elements share common outputs a single stage involves a differential output and four inputs representing a pair of differential inputs. Thus, the configuration is that of a pair of differential amplifiers having a common output. Ideally, the pair of amplifiers will have the same transconductance which can be varied by a common control.